THE CAMBRIAN PERIOD. 



85 



anterior of these divisions is a shield or buckler which covers 

 the head ; the second or middle portion is composed of mov- 

 able rings covering the trunk ("thorax"); and the third is a 

 shield which covers the tail or " abdomen. " The head-shield 

 (fig. 31, e) is generally more or less semicircular in shape; and 

 its central portion, covering the stomach of the animal, is usu- 

 ally strongly elevated, and generally marked by lateral furrows. 

 A little on each side of the head are placed the eyes, which 

 are generally crescentic in shape, and resemble the eyes of 

 insects and many existing Crustaceans in being " compound, " 

 or made up of numerous simple eyes aggregated together. 

 So excellent is the state of preservation of many specimens of 

 Trilobites, that the numerou t> individual lenses of the eyes 

 have been uninjured, and as many as four hundred have been 

 counted in each eye of some forms. The eyes may be sup- 

 ported upon prominences, but they are never carried on mov- 



Flg. 31. Cambrian Trilobites: a, Paradoxides Bohem{cus,ve<l\ice<!L In size; b, Ellip- 

 tocephalua Hoffl; c, Sao hirsuta; d, Conocoryphe Sultzeri (all the above, together with 

 fig. g, are from the Upper Cambrian or "Primordial Zone" of Bohemia); e, Head-shield 

 of Dikellocephalus Celticus, from the Llngula Flags of Wales;/, Head-shield of Cono- 

 coryphe Mattfiewi, from the Upper Cambrian (Acadian Group) of New Brunswick; g, 

 Agnostus rex, Bohemia; h. Tail-shield of Dikellocephalus Minnesotenaia, from the 

 Upper Cambrian (Potsdam Sandstone) of Minnesota. (After Barrande, Dawson, Salter, 

 and Dale Owen.) 



